The present invention relates generally to a novel illumination system, and particularly to an illumination system which provides excellently well-collimated light for exposure, without any irradiance reduction in the peripheral area of an object to be illuminated, so that it is particularly useful for an exposure system applicable to exposure of printed circuit boards, integrated circuits or the like, for an exposure system applicable to contact exposure apparatus for plate making, step-and-repeat machines or the like, and for an illumination system applicable to copy machines or the like.
It is well known that several illumination systems have been used in an exposure apparatus for use in contact exposure and proximity exposure for printed circuit boards. One of these illumination systems is shown in U.S. Pat. No. 4,389,115 patented to Richter, which is indeed advantageous for the purpose of improving the uniformity of light intensity on an object to be illuminated, in comparison with conventional illumination systems theretofore. However, Richter patent requires an elaborately curved collimation mirror, which is positioned as remotely as possible from a secondary light source.
Generally, in order to correct the non-uniformity of the luminous intensity distribution of a light source, a secondary light source is formed by an appropriate optical system, light from which is converted into collimated light by a collimation system. In such optical system, it is needed to make the intensity distribution of the incident light to the collimation system uniform. For this reason, the illumination system disclosed in Richter patent is arranged so that the collimation mirror is positioned as remotely as possible from the secondary light source. Furthermore, the Richter patent teaches that one should adopt an elaborately curved collimation mirror having a long focal length, for the purpose in abating the spherical aberration.
For these reasons, conventional illumination systems heretofore have a long optical path, which makes an illumination apparatus incorporating such illumination systems large in size.